Direct Dimesitylborylation of Benzofuran Derivatives by an Iridium-Catalyzed C?H Activation with Silyldimesitylborane

Article abstract of DOI:10.1002/chem.201903776

Direct dimesitylborylation of benzofuran derivatives by a C?H activation catalyzed by an iridium(I)/N-heterocyclic carbene (NHC) complex in the presence of Ph₂MeSi-BMes₂ afforded the corresponding dimesitylborylation products in good to high yield with excellent regioselectivity. This method provides a straightforward route to donor–(π-spacer)–acceptor systems with intriguing solvatochromic luminescence properties.

Full text of DOI:10.1002/chem.201903776

A Journal of
Accepted Article
Title: The Direct Dimesitylborylation of Benzofuran Derivatives via an
Iridium-Catalyzed C–H Activation with Silyldimesitylborane
Authors: Ryosuke Shishido, Ikuo Sasaki, Tomohiro Seki, Tatsuo
Ishiyama, and Hajime Ito
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To be cited as: Chem. Eur. J. 10.1002/chem.201903776
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The Direct Dimesitylborylation of Benzofuran Derivatives via an
Iridium-Catalyzed C–H Activation with Silyldimesitylborane
Ryosuke Shishido,[a] Ikuo Sasaki,[b] Tomohiro Seki,[a,c] Tatsuo Ishiyama,[a] and Hajime Ito*[a,c]
Abstract: The direct dimesitylborylation of benzofuran derivatives via
a C–H activation catalyzed by an iridium(I)/N-heterocyclic carbene
a) Synthetic routes to aryldimesitylboranes from organohalides
(NHC) complex in the presence of Ph2MeSi–BMes2 afforded the
LG–BMes2
metallation
corresponding dimesitylborylation products in good to high yield with
excellent regioselectivity. This method provides a straightforward
route to donor-(π-spacer)-acceptor systems with intriguing
solvatochromic luminescence properties.
(A)
(LG = X, OR)
Mes
B
Mes
R
R
2 MesLi
B(OR')3
metallation
X
(B)
(X = halogen)
(C)
Ph2MeSi–BMes2 1
Na(OtBu), 50 ºC, 24 h
Boron-containing
π-conjugated
compounds
such
as
triarylboranes have attracted much attention due to their unique
photophysical and electronic properties. These properties arise
from the pπ-π* conjugation between the vacant p-orbital of the
boron atom and the π*-orbital of the connected carbon-based π-
conjugated moieties.[1] The dimesitylboryl (BMes2) group is
frequently used in this context due to its high π-electron-accepting
abilities and its desirable stability in air. However, methods to
introduce BMes2 groups into aromatic compounds remain highly
limited. A popular method for the transfer of BMes2 groups is the
nucleophilic substitution of BMes2 electrophiles (Mes2B–X; X =
halogen or OR) with organometallic reagents (Ar–M; M = Li, Mg)
generated from organic halides via halogen-metal exchange
[Scheme 1a, (A)].[2,3] BMes2 groups can also be introduced into
aromatic compounds by the reaction of aryl boronic acid esters
with MesLi [Scheme 1a, (B)].[2,4] Recently, our group has reported
b) Concept of this study:
Direct introduction of BMes2 through C–H activation
Mes
H
B
C–H activation
Mes
Ar
Ar
one-step introduction
this work
H
H
aromatic componds
triarylboranes
X
Ar
conventional
methods
multi-step
synthesis
organohalide precursors
c) This work: C–H dimesitylborylation of benzofurans
the
direct
dimesitylborylation
of
aryl
halides
with
Ph2MeSi–BMes2 (1)
Ir(I) cat./IMes·HCl
silyldimesitylborane Ph2MeSi–BMes2 and Na(OtBu), i.e., a base-
mediated borylation with silylborane (BBS reaction) [Scheme 1a,
(C)].[5,6] Although the corresponding dimesitylborylation products
are obtained in good yield using both methods, a stoichiometric
R
R
B
H
alkoxide base
1,4-dioxane, 120 ºC
O
O
benzofuran 2
3
amount of
base or
organometallic reagent is needed.
Furthermore, in both reactions, the availability of the BMes2
compounds relies on the availability of the preceding
organohalide precursors, which are often difficult to access,
especially in case of highly functionalized organic halides. Hence,
more efficient and direct methods are required to improve the
availability of triarylboron compounds for the introduction into
organic compounds.
Scheme 1. a) Synthetic routes to aryldimesitylboranes from organohalides. b)
Schematic illustration of the concept of this study. c) This work: Direct
dimesitylborylation of benzofurans via an iridium-catalyzed C–H activation.
Iridium-catalyzed aromatic C–H borylations[7,8] using diboron
or silylborane compounds[9] represent a powerful method for the
preparation of arylboron compounds, and have often been used
for the synthesis of organic materials,[10] natural products,[11] and
fine chemicals.[11] This method enables the direct borylation of C–
[a]
[b]
R. Shishido, Dr. T. Seki, Dr. T. Ishiyama, Prof. Dr. H. Ito
Division of Applied Chemistry, Graduate School of Engineering
Hokkaido University
Kita 13 Nishi 8 Kita-ku, Sapporo, Hokkaido 060-8628 (Japan)
E-mail: hajito@eng.hokudai.ac.jp
H
bonds in aromatic compounds without requiring any
halogenated intermediates and providing the corresponding
arylboronates in high yield with excellent regioselectivity.
Therefore, the direct introduction of BMes2 into aromatic
compounds via iridium-catalyzed C–H borylations may potentially
provide a new method for the synthesis of boron-containing
organic compounds with a triarylboron structure [Scheme 1b].
This method thus allows a late-stage introduction of the BMes2
group,[12] which would be advantageous especially for the
compilation of compound libraries with complicated structures for
screening purposes. However, previous examples of the
synthesis of triarylboranes with this strategy have not yet been
reported. Most examples of C–H borylations introduce boronate
Dr. I. Sasaki
Department of Chemistry and Bioscience, Faculty of Science and
Technology, Aoyama Gakuin University
5-10-1, Fuchinobe, Chuo-ku, Sagamihara 252-5258 (Japan)
[c]
[d]
Prof. Dr. H Ito, Institute for Chemical Reaction Design and Discovery
(WPI-ICRD), Hokkaido University
Kita 21, Nishi 10, Kita-ku, Sapporo 001-0021, Japan
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under: 0000-0003-3852-
6721
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standard. The isolated yield of 3a is shown in parentheses. [c] K(OtBu) (Y
mol %) was added to the reaction mixture.
groups [B(OR)2] such as B(pin), and only one example for the
introduction of B(9-BBN)2 (9-BBN: 9-borabicyclo[3.3.1]nonan)
has been reported.[13] Herein, we report the first example of the
direct introduction of BMes2 into heteroaromatic benzofuran
derivatives (2) via an iridium-catalyzed C–H activation using
Ph2MeSi–BMes2 (1)[14] as the borylation reagent [Scheme 1c].
The C–H dimesitylborylation proceeds smoothly in the presence
of [Ir(Cl)(coe)2]2/IMes (IMes: 1,3-dimesitylimidazol-2-ylidene; coe:
cyclooctene) to afford the corresponding products in good to high
We initially investigated the optimization of the reaction
conditions for the Ir-catalyzed C–H dimesitylborylation of
benzofuran (2a) with 1 (Table 1),[15] starting by screening a series
of different ligands (Table 1; entries 1–6). For that purpose, a 1,4-
dioxane solution of 1 and 2a was heated to 120 ºC in the presence
of [Ir(OMe)(cod)]2 (2.5 mol %; cod: 1,5-cyclooctadiene) and a
variety of different ligands. The use of IMes×HCl/K(OtBu) (10
mol %) furnished 3a in 48% yield (Table 1; entry 1). The silylated
product was also detected as a side product. The presence of
K(OtBu) was required to generate the Ir–NHC complex catalyst in
situ. 4,4-Di-tert-butyl-2,2’-bipyridine (dtbpy; 5.0 mol %) and
3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen; 5.0 mol %),
which are effective ligands for typical Ir-catalyzed C–H borylation
reactions, did not facilitate the dimesitylborylation of the C–H
bonds of 2a (entries 2 and 3). Using monodentate (10 mol %) or
bidentate (5.0 mol %) phosphines such as triphenylphosphine
yield
with
excellent
regioselectivity.
Some
of
the
dimesitylborylation products obtained exhibit
a
pronounced
solvatochromic luminescence properties due to their donor-(π-
spacer)-acceptor (D-π-A) structure.
Table 1. Optimization of the reaction conditions for the iridium-catalyzed C–H
borylation of benzofuran (2a) using 1.[a]
H
O
2a (5.0 equiv)
(PPh3),
tri-tert-butylphosphine
(PtBu3)
and
1,2-
Ph
Me Si B
Ph
Ir(I) precursor (X mol %)
ligand (Y mol %)
bis(dicyclohexylphosphino)ethane (dcpe) did not generate 3a
efficiently (entries 4–6; 0–13%). These results suggest that the
presence of NHC ligands is crucial to promote the
dimesitylborylation of 2a efficiently. Therefore, we further
examined various Ir(I) precursors and NHC ligands (Table 1;
entries 7–15). In the presence of [Ir(Cl)(coe)2]2 (2.5 mol %) and
IMes×HCl/K(OtBu) (10 mol %), 3a was obtained in 69% yield
(entry 7). The use of [Ir(Cl)(cod)]2 (2.5 mol %) resulted in a slightly
lower yield of 3a (58% yield; entry 8). Using Crabtree's catalyst
[Ir(cod)(Py)(PCy3)][PF6] (5.0 mol %; Py: pyridine) furnished 3a in
merely 5% yield (entry 9). The cationic iridium catalyst
B
O
1,4-dioxane, 120 ºC, 24 h
1 (1.0 equiv)
3a
Cl
N
Cl
Cl
N
iPr
iPr
N
N
N
N
Cy
Cy
iPr
iPr
IMes·HCl, SIMes·HCl
IPr·HCl
ICy·HCl
Entry
1
Ir(I) precursor [X mol %]
[Ir(OMe)(cod)]2 (2.5)
[Ir(OMe)(cod)]2 (2.5)
[Ir(OMe)(cod)]2 (2.5)
[Ir(OMe)(cod)]2 (2.5)
[Ir(OMe)(cod)]2 (2.5)
[Ir(OMe)(cod)]2 (2.5)
[Ir(Cl)(coe)2]2 (2.5)
ligand [Y mol %]
IMes×HCl (10)[c]
dtbpy (5.0)
Yield[b]
[Ir(cod)2][BArF ] [5.0 mol %; ArF: 3,5-bis(trifluoromethyl)phenyl]
4
showed good reactivity (65% yield; entry 10), similar to that of
[Ir(Cl)(coe)2]2. Diminishing the catalyst loading {[Ir(Cl)(coe)2]2 (2.5
mol %) and IMes×HCl/K(OtBu) (5.0 mol %)} also afforded 3a in
high yield (75% 1H NMR yield; 59% isolated yield; entry 11). The
use of other NHC ligands such as SIMes, IPr, or ICy decreased
the yield of 3a (SIMes: 64%; IPr: 10%; ICy: 11%; entries 12–14).
3a was not obtained in the absence of IMes (entry 15). Moreover,
using H–BMes2 instead of Ph2MeSi–BMes2 did not furnish any 3a
(Table S5).[16] Therefore, the optimal conditions to obtain a
maximum of 3a involve [Ir(Cl)(coe)2]2 (2.5 mol %) and IMes (5.0
mol %).[17] Under these conditions, the silylation side product was
formed in 29% yield.
48
0
2
3
Me4phen (5.0)
PPh3 (10)
0
4
0
5
PtBu3 (10)
13
0
6
dcpe (5.0)
7
IMes×HCl (10)[c]
IMes×HCl (10)[c]
IMes×HCl (10)[c]
IMes×HCl (10)[c]
IMes×HCl (5.0)[c]
SIMes×HCl (5.0)[c]
IPr×HCl (5.0)[c]
ICy×HCl (5.0)[c]
-
69
58
5
8
[Ir(Cl)(cod)]2 (2.5)
With the optimized conditions in hand, we proceeded to
investigate the substrate scope for this C–H dimesitylborylation
(Table 2).[18] 5-Methylbenzofuran (2b) reacted with 1 to give the
corresponding borylation product (3b) in high yield (73%).
Substrates bearing two methyl groups such as 5,7-dimethyl- (2c),
4,6-dimethyl- (2d), and 4,7-dimethylbenzofuran (2e) afforded 3c,
3d, and 3e, respectively, in good yield (3c: 77%; 3d: 69%; 3e:
62%). The reaction of 5-chlorobenzofuran (2f) proceeded
smoothly without any side reactions involving the C–Cl bond,
even though some transition-metal catalysts show high reactivity
for the cleavage of such C–Cl bonds. Unfortunately, furan
substrate 2g did not readily engage in the C–H
dimesitylborylation.[19] The optimized catalyst system also worked
well for benzofuran derivatives bearing aromatic rings at the 5-
position (2h–2k) to furnish the corresponding products in good
9
[Ir(cod)(Py)(PCy3)][PF6] (5.0)
[Ir(cod)2][BArF4] (5.0)
[Ir(Cl)(coe)2]2 (2.5)
10
11
12
13
14
15
65
75 (59)
64
10
11
0
[Ir(Cl)(coe)2]2 (2.5)
[Ir(Cl)(coe)2]2 (2.5)
[Ir(Cl)(coe)2]2 (2.5)
[Ir(Cl)(coe)2]2 (2.5)
[a] Conditions: 1 (0.10 mmol), 2a (0.50 mmol), [Ir(OMe)(cod)]2 (0.0025 mmol),
and ligand (0.005 or 0.01 mmol) in 1,4-dioxane (0.5 mL) at 120 ºC for 24 h. [b]
1H NMR yield of 3a. 1,1,2,2-Tetrachloroethane was used as an internal
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yield (3h: 62%; 3i: 68%; 3j: 67%; 3k: 58%). The reaction of 9-
carbazolyl-substituted benzofuran 2l afforded 3l in good yield
(66%). The molecular structure of 3l was confirmed by a single-
crystal x-ray diffraction analysis (Figure 1). This result indicates
high reactivity of this C–H dimesitylborylation only toward
benzofuran derivatives. This unique reactivity enabled the site-
selective C–H dimesitylborylation of the benzofuran moiety in 2m,
which bears an additional furan substituent (Scheme 2).
Table 2. Substrate scope for the Ir-catalyzed C–H dimesitylborylation of
substituted benzofuran derivatives.[a]
Ph2MeSi–BMes2 1 (1.0 equiv)
[IrCl(coe)2]2 (2.5 mol %)
Figure 1. Crystal structure of 3l with thermal ellipsoids at 50% probability. Color
code: grey: carbon; white: hydrogen; pink: boron; red: oxygen; pale purple:
nitrogen.
IMes·HCl/K(OtBu) (5.0 mol %)
R
R
B
H
O
O
1,4-dioxane, 120 ºC, 24 h
2 (5.0 equiv)
3
BMes2
BMes2
BMes2
O
O
O
H
H
1 (1.0 equiv)
3b, 73 (60)%
O
3c, 77 (60)%
3d, 69 (64)%
O
H
B
optimized
conditions
O
O
Cl
BMes2
BMes2
BMes2
2m (5.0 equiv)
3m, 55 (36)%
O
O
O
C5H11
Scheme 2. Site-selective C–H dimesitylborylation of benzofuran 2m, which
bears an additional furan substituent.
3e, 62 (52)%
3f, 74 (42)%
3g, 16%[b]
Me2N
Dimesitylborylation product 3i exhibited pronounced
solvatochromic luminescence properties due to the D-π-A
structure, which includes the benzofuran moiety and the phenyl
spacer (Figure 2). The absorption and emission spectra of 3i were
measured in various solvents. Two absorption maxima (λabs = 290
and 350 nm) were observed that were relatively unaffected by the
solvent polarity (Figure S1). Yet, we obtained seven different
emission spectra for 3i (lex = 365 nm) in seven different solvents
(hexane, toluene, Et2O, CH2Cl2, THF, acetone and CH3CN)
(Figure 2). All spectra of 3i exhibited a broad band with a distinct
emission maximum (λem, max) ranging from 455 nm to 722 nm
(hexane: λem, max = 455 nm, Φem = 29.4%; toluene: λem, max = 504
nm, Φem = 43.5%; Et2O: λem, max = 536 nm, Φem = 43.5%; CH2Cl2:
BMes2
BMes2
O
O
3h, 62 (40)%
3i, 68 (60)%
MeO
BMes2
O
3j, 67 (57)%
OMe
MeO
N
BMes2
BMes2
O
O
λem, max = 587 nm, Φem = 59.4%; THF: λem, max = 602 nm, Φem
=
50.0%; acetone: λem, max = 667 nm, Φem = 29.4%; CH3CN: λem, max
3k, 58 (53)%
3l, 66 (59)%
= 722 nm, Φem = 43.5%), which demonstrates that λem,
max
changes with the solvent polarity. These results indicate that 3i
shows properties that are characteristic for push-pull
solvatoluminescent dyes. Therefore, the Ir-catalyzed C–H
dimesitylborylation of substituted benzofuran derivatives
described in this article permits the rapid construction of the D-π-
A systems found in such dyes.
[a] Conditions: 1 (0.10 mmol), 2 (0.50 mmol), [Ir(Cl)(coe)2]2 (0.0025 mmol),
and lMes·HCl (0.01 mmol) in 1,4-dioxane (0.5 mL) at 120 ºC. The yield of
the products was determined by 1H NMR analysis using 1,1,2,2-
tetrachloroethane as the internal standard. Isolated product yield values are
shown in parentheses. [b] Identified based on 1H NMR spectroscopy and
GC-MS spectrometry.
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it seems feasible to assume that the Ir(III) complexes [Ir(B)n(Si)3–
n] (n = 1 or 2) are generated in situ and act as actual active
catalytic species.[9, 23] At present, we speculate that the generation
of the monoboryliridium(I) complex A would be favored relative to
that of [Ir(B)n(Si)3–n] (n = 1 or 2) due to the presence of bulky boryl
and silyl groups.[25]
IMes IrCl(coe)2
Ph2MeSi BMes2
1
Ph2MeSi Cl
IMes Ir BMes2
3a'
Silylation path
Ph2MeSi
(I)
H
H
O
A
Ph2MeSi BMes2
2a
Oxidative addition
1
Figure 2. Photograph and emission spectra of 3i in various solvents under
irradiation from UV light (λex = 365 nm; 2.5 mM).
IMes Ir
(I)
H
O
H
IMes Ir BMes2
C
(III)
B
Silylation products were detected as the main side products
of the dimesitylborylation reactions. To gain insight into the
mechanism of their generation, a control experiment was carried
out (Scheme 3). The reaction between 2a and Ph2MeSi–H instead
of Ph2MeSi–BMes2 led to the formation of the silylation product
3a' in 29% yield.[20–22] This result suggests that Ph2MeSi–H, which
would be produced as a by-product in the borylation reaction,
reacts with 2a to give the silylation side product.
BMes2
Reductive elimination
O
3a
Scheme 4. Possible reaction mechanism for the Ir(I)-catalyzed C–H
dimesitylborylation of benzofuran derivatives.
In summary, we have developed a novel method for the C–H
dimesitylborylation of benzofuran derivatives, which is the first
example of a direct dimesitylborylation of aromatic compounds
through C–H activation using a catalyst system based on an
Iridium(I)/N-heterocyclic carbene complex. These reactions afford
the corresponding dimesitylborylation products in good to high
yield with excellent regioselectivity. This method thus enables the
one-step introduction of luminescent functionality into non-
luminescent heteroarenes at a late stage in their synthesis, which
should significantly promote the synthesis of novel organic
materials. Detailed mechanistic studies and efforts to expand the
substrate scope are currently in progress in our laboratory.
Ph2MeSi–H(1.0 equiv)
[IrCl(coe)2]2 (2.5 mol %)
IMes·HCl/K(OtBu) (5.0 mol %)
H
SiMePh2
3a': 29%
1,4-dioxane, 120 ºC, 24 h
O
O
Si–H full conversion
2a (5.0 equiv)
Scheme 3. Control experiment using PhMe2Si–H instead of 1.
Based on previous mechanistic studies[9, 23] for the Ir-catalyzed
C–H
borylation
of
aromatic
compounds
with
bis(pinacolate)diboron and silylborane, and considering the
results of our control experiments, we would like to propose a
plausible reaction mechanism for this C–H dimesitylborylation
(Scheme 4). The NHC-Ir(I) complex generated in situ could
Acknowledgements
This work was financially supported by JSPS KAKENHI Grant
Numbers JP15H03804, JP17H06370, JP18H03907, and
Program for Leading Graduate Schools. The Institute for
Chemical Reaction Design and Discovery (WPI-ICRD) was
established by the World Premier International Research Initiative
(WPI), MEXT, Japan.
initially
react
with
Ph2MeSi–BMes2
(1)
to
afford
monoboryliridium(I) complex A as an active catalyst species. The
subsequent oxidative addition of a C–H bond at the 2-position in
2a to complex
A would produce Ir(III) complex B. This
regioselectivity could be assigned to the high acidity of the C–H
bond in the benzofuran ring.[24] Reductive elimination of the
desired dimesitylborylation product (3a) would lead to the
formation of Ir(I)-hydride complex C. Finally, the oxidative addition
of 1 to complex C, followed by a reductive elimination of Ph2MeSi–
H, would regenerate Ir(I) complex A. Additionally, Ph2MeSi–H
would rapidly engage in a side reaction of 2a to provide the
silylated benzofuran 3a’ via a C–H activation process. Moreover,
Keywords: C–H borylation • Iridium catalyst • Dimesitylboryl
group • Silyldimesitylborane • Benzofuran
[1]
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Wakamiya, S. Yamaguchi, Bull. Chem. Soc. Jpn. 2015, 88, 1357; b) F.
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[15] For further optimizations of the reaction conditions, see the supporting
information.
[2]
[3]
For selected reviews, see: D. G. Hall, Boronic Acids: Preparation and
Applications in Organic Synthesis Medicine and Materials, 2nd ed.,
Wiley-VCH, Weinheim, 2011.
[16] Preliminary experiments for the reaction using H–BMes2 instead of
Ph2MeSi–BMes2 were also conducted. For details, see the supporting
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10.1002/chem.201903776
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COMMUNICATION
Ryosuke Shishido, Ikuo Sasaki,
Tomohiro Seki, Tatsuo Ishiyama, Hajime
Ito*
Ph2MeSi–BMes2
[IrCl(coe)2]2/IMes
1
R
R
H
B
C–H Activation
O
O
Page No. – Page No.
The first direct C–H dimesitylborylation
of benzofuran derivatives
The Direct Dimesitylborylation of
Benzofuran Derivatives via an
Iridium-Catalyzed C–H Activation with
Silyldimesitylborane
The first dimesitylborylation of benzofuran derivatives via an Ir-catalyzed C–H
activation has been accomplished. This reaction provides direct access to donor-
(π-spacer)-acceptor systems with intriguing luminescence properties.
This article is protected by copyright. All rights reserved.